As an example, advances in structural adhesives have permitted engineers to contemplate the use of bonded joints in areas that have long been dominated by mechanical fasteners and welds. The deployment of bonded joints generally requires the use of sensitive nondestructive inspection techniques to ensure the continued integrity of the bond joint.
For example, the use of adhesive bonding in automobile bodies is increasing every year for various reasons. However, current quality control of these joints relies primarily on the robust control of the adhesive preparation and application. These controls may include machine vision inspection of the applied adhesive bead. However, there is no practical method currently available to test the overall quality of the final joints other than destructive testing. Nondestructive testing would reduce this scrap, increase the rate of testing, and improve the reliability of adhesive joints. Within the automotive manufacturing arena, nondestructive inspections would be beneficial in the body shop before the adhesive is cured. Here, discrepant joints could be repaired. However, inspections would also be beneficial at the end of the assembly line to ensure the quality of the entire assembled, cured, and painted product. Nondestructive inspections are also seen as a major cost savings for accelerating engineering and environmental testing, ramp-up to production, and monitoring the long term performance of the joints.
In the automobile example, individual adhesive joints can be long (approaching 1 m) and relatively narrow (typically 10-25 mm). Moreover, the sum total of the adhesive joints may be over 100 m in length in a single vehicle. This requires a practical inspection device that can transverse long bead or flange structures at speeds preferably over 1 m/min. To ensure the adhesive joint strength, the bead width, thickness, location on flanges, and the state of cure are all desirable inspection features.
In the automobile example, access to the back side of the material being inspected is not always possible due to geometry constraints and other obstructions. This limits the applicability of some nondestructive techniques, for example through-transmission ultrasound (since the receiving transducer requires back side access) or radiography (since the digital panel or film requires back side access). While pulsed thermography can be a single sided technique, an emissive coating must usually be applied to the surface of metal parts prior to an inspection. The coating residue must be completely removed before the automobile is painted which adds complexity, risk, and cost to the inspection process. Eddy current inspection is also not viable because the adhesive is nonconductive.
One possible technology for nondestructive inspection is pulse-echo ultrasound. This uses short bursts of high frequency sound waves that are introduced into the material for the detection of surface and subsurface flaws in the material (e.g., composites, metals, plastics, ceramics, etc.). The ultrasonic waves are generated by an ultrasonic probe (transmitter). The ultrasonic wave interacts with the material and the resulting wave is detected by an ultrasonic probe (receiver). Water, gel or other fluids are typically applied between the probe(s) and the material to couple the sound since it cannot travel as well through air. If sufficient coupling is achieved, the ultrasonic waves travel through the material with some attenuations and are reflected at interfaces. The reflected beam is displayed on an instrument and then analyzed to define the presence and location of adhesive and flaws (e.g., disbonds). Complete reflection, partial reflection, scattering, or other detectable effects on the ultrasonic waves can be used as the basis of flaw detection. In addition to wave reflection, the time of transit through the test piece can be used to assess adhesive bond-line thickness.
For good results, it is important to maintain good coupling along the entire ultrasonic wave path from transmitter to receiver. This can be a significant challenge when the probe encounters complex curvature and uneven inspection surfaces, for example from spot welds and spot weld expulsions which are common in automobile design and production. In addition, if a fluid is used to enhance the coupling, for automotive applications, the fluid couplant preferably does not leave a residue (such as those left by gels), is not toxic, and does not require large drain or tank systems.
Thus, there is a need for ultrasound deployment devices that can accommodate the rapid inspection of complex curvatures and shapes, and/or uneven inspection surfaces. There is a further need for such devices that do not require access to the back side of the material being inspected. In addition, there is a need for such devices to be small enough that they can be used to inspect narrow flanges or joints and fit into small hard to reach or confined areas, some of which can be vertical or inverted. There is a further need for devices that provide for reduced leakage/loss of fluid couplants while minimizing fluid turbulence within the device that can degrade the ultrasonic signal. It is also desirable to produce two-dimensional images of the inspection area (e.g. C-Scan images) as opposed to one-dimensional inspection methods (e.g. A or B-scan imaging).